Hybrid vehicles are demanded more and more recently from the standpoint of social demands such as low fuel consumption and low exhaust emission. In hybrid vehicles, which are presently available in the market, as disclosed in JP 2001-329884A, an internal combustion engine, a first motor generator (MG) used primarily as a generator and a second motor generator (MG) used primarily to drive wheels are provided. The crankshaft of the internal combustion engine is coupled to a planet carrier of a pinion gear of a planetary gear set, which is a power distribution mechanism. The first MG is coupled to a sun gear of the planetary gear set. A wheel drive shaft and the second MG are coupled to a ring gear of the planetary gear set.
According to this type of hybrid vehicle, a fail-safe operation is performed when a predetermined abnormality (for example, abnormality in an engine system or abnormality in a first MG system) arises. As the fail-safe operation, the vehicle is driven by only drive power of the second MG while stopping the operations of the engine and the first MG. Thus, the operation mode of the vehicle is changed to a limp-home travel mode, which is a fail-safe mode or a motor drive mode.
However, if the operation of the engine is stopped immediately at the time of changing the vehicle operation mode to the limp-home travel mode in response to occurrence of abnormality in the engine system or the first MG system while traveling at high speeds, the engine rotation speed falls and the engine stops while the travel speed of the vehicle is still relatively high. In this case, the difference between the rotation speed of the ring gear coupled to the wheel side and the rotation speed of the carrier coupled to the engine side becomes large in the planetary gear set. The pinion gear rotation speed is likely to exceed an allowable maximum limit value. If the pinion gear rotation speed exceeds the allowable maximum limit value, the pinion gear will be burned out and the planetary gear set will fail and become inoperative. This will result in a secondary failure.
If the pinion gear rotation speed exceeds the allowable maximum limit value and the planetary gear set fails causing the secondary failure when the vehicle operation mode is changed to the limp-home travel mode due to abnormality in the first MG system (for example, first MG, inverter for first MG or the like), not only the first MG system (for example, first MG or inverter for first MG), which is the original cause for the abnormality, but also the planetary gear set need be replaced with new system and new set in a repair work. As a result, repair cost will greatly increase.
To meet requirements for reduction of the number of component parts (reduction of assembling work), downsizing and low cost, drive power transfer system parts such as the first MG and the planetary gear set are integrated into a single transfer assembly unit and mounted in a vehicle. If the pinion gear rotation speed exceeds the allowable maximum limit value and the planetary gear set fails causing the secondary failure when the vehicle operation mode is changed to the limp-home travel mode due to abnormality in the inverter of the first MG in the first MG system, not only the inverter of the first MG system, which is the original cause for the abnormality, but also the transfer assembly unit (the first MG, the planetary gear set and the like) need be replaced with new ones in a repair work. As a result, repair cost will greatly increase.